1. Field of the Invention
The present invention relates to cable integrity in a network. More particularly, the present invention relates to determining the length of cables linking points in a network and maintaining the integrity and security of those links.
2. The Background
Computer networks using cables to link devices such as computers are common and the number of such networks is growing. Ethernet, as specified by Institute of Electrical and Electronics Engineers (IEEE) Standard 802.3 (1985), and Token Ring, IEEE Standard 802.5 (1985), are just two of the many standards for such networks known in the art. In addition to these standards, there are also many proprietary or non-standard network configurations. In most cases, there are well-defined specifications for the cables and the arrangement of the devices in such networks. There are also many various types of cables known in the art, such as coaxial, shielded twisted pairs, unshielded twisted pairs and fiber optic. Each of these cable types can typically be further subdivided, such as the Electronic Industry Association (EIA/Telecommunications Industry Association (TIA) Category system for unshielded twisted pair cables. There may be an overlap between network configurations and cable specifications, a given cable may support more than one configuration, and vice versa.
The trend in computer networks is clearly towards faster rates of data transfer. This trend exists both in response to the needs of users and because of a push by the industry to make available the technology that is possible, ideally ahead of the competition. Coexisting with this trend towards faster networks is an existing infrastructure of installed hardware, such as cables routed through and between buildings. It would be advantageous to use this existing infrastructure where possible, thereby minimizing the total costs to upgrade the capabilities of a network.
Existing network cables were often installed with a particular type of network and cable specification in mind. For example, many existing cables were installed with 100-150 meters or less between devices in order to conform to Ethernet 10Base-T and/or Ethernet 100Base-T standards. These Ethernet Base-T systems typically use only two pairs of conductors while many of the cables used for this purpose contain four pairs of conductors. It is quite possible to envision that a new network protocol, not defined today, will make use of these extra pairs of conductors to increase the speed of data transmissions. It is also possible that a new protocol may use these cables, originally installed for Ethernet 10Base-T or Ethernet 100Base-T systems, but require a shorter distance between devices in order to facilitate faster data transfers. It would be advantageous to have a way of easily measuring the length of existing network cables when making a decision concerning whether or not to use the cable to support a new network protocol. Ideally, it would not require physically measuring the lengths of cables that may not be easily accessible, such as those behind walls, in ceilings and shafts, and under floors.
There are methods known in the art that locate hidden cables using tone generators. These methods still require tracing the route of an existing cable through the walls in order to physically measure the length. There are also devices such as Time Domain Reflectometers (TDRs) known in the art that measure cable lengths by sending a signal down a cable and measuring the reflection back. TDRs only use one end of a cable and rely on the weak signal reflected back from a discontinuity at the other end of the cable to estimate the length of a cable, sometimes leading to relatively poor estimates of length. Accurate TDRs exist, although they are expensive. It would be advantageous to measure the signal directly, rather than the weak reflection, in order to inexpensively measure the length of a cable. It would also be advantageous to have a cable measurement system integrated into the computer network so that software may invoke it in order to detect unauthorized taps in the cable and for other purposes.
A method and apparatus for measuring the length of a cable link in a computer network sends a test signal through the cable and examines how the signal is received. Measurements of the signal transit time, decrease in signal amplitude, and decrease in signal power are three techniques that may be used to measure cable lengths, individually or in combination. One or more transceivers are connected to the cable to send and receive the test signals, so that there is no need for access to the cable, except at the ends. Cable lengths may be calculated for both electrically conductive and fiber optic type cables. The length of an existing cable may be compared with a maximum length allowed by a particular data transfer protocol to verify compliance, or it may be compared against a previously stored value for purposes such as security checks to verify that the cable properties have not been altered as they might be if the cable were tapped, rerouted, shortened, extended or otherwise changed.